Microporous polyolefin membranes are widely used in separators for lithium batteries, etc., electrolytic capacitor separators, steam-permeable, waterproof clothing, various filters, etc. When the microporous polyolefin membranes are used as battery separators, their performance largely affects the performance, productivity and safety of batteries. Particularly lithium ion battery separators are required to have excellent mechanical properties and permeability, as well as a function of closing pores at the time of abnormal heat generation to stop a battery reaction [shutdown (SD) property], and a function of keeping the shape of the separators even at high temperatures to prevent the direct reaction of cathode materials and anode materials (dimensional stability), thereby preventing the heat generation, ignition, explosion, etc. of batteries that can occur by the short-circuiting of external circuits, overcharging, etc.
Japanese Patent 3235669 discloses a battery separator having excellent dimensional stability and SD property, which comprises at least one first layer made of a polymer selected from low-density polyethylene, an ethylene-butene copolymer and an ethylene-hexene copolymer, and at least one second layer made of a polymer selected from high-density polyethylene, ultra-high-molecular-weight polyethylene and polypropylene.
Japanese Patent 3589778 discloses a three-layer, porous laminate membrane comprising a porous polypropylene membrane sandwiched by porous membranes made of a mixture of polyethylene and polypropylene, the highest temperature of the membrane being equal to or lower than the melting point of polyethylene+20° C. when its temperature is elevated at a speed of 10 to 50° C./second by the resistance heat generation of an electrolytic solution impregnated into the membrane, that is caused by applying AC voltage to electrodes disposed on both surfaces of the electrolytic-solution-impregnated membrane, wherein the resistance of the porous membrane increases to such a level of shutting current immediately when polyethylene is melted.
WO 2004/089627 proposes a microporous polyolefin membrane having excellent permeability, high-temperature strength, high-temperature retention and safety as well as a low SD temperature and a high short-circuiting temperature, wherein the membrane comprises polyethylene and polypropylene as indispensable components and is constituted by two or more laminate films, wherein a polypropylene content in at least one surface layer is more than 50% by mass and 95% or less by mass, and wherein a polyethylene content in the entire membrane is 50% by mass to 95% by mass.
However, the microporous membrane containing polypropylene in at least one surface layer is poor in film formability and thickness uniformity. Specifically, when the microporous membrane is slit, a large amount of polypropylene powder is detached, resulting in defects such as pinholes and dots in the microporous membrane. The use of microporous membranes with poor thickness uniformity as batteries separators is likely to cause safety problems such as short-circuiting and low compression resistance, and a low yield leading to poor productivity of batteries. Microporous membranes containing polypropylene in surface layers also suffer from high SD temperatures and low SD speeds.
JP2002-194132 A proposes a microporous polyolefin membrane containing polyethylene and polypropylene and having excellent flatness and compression properties, wherein the microporous polyolefin membrane comprises polypropylene having MFR of 2.0 or less, and polyethylene having a ratio (mass-average molecular weight/number-average molecular weight) of 8 to 100, and wherein the polypropylene content is 20% or less by mass.
JP2004-196870 A proposes a microporous polyolefin membrane, having well-balanced thickness uniformity, mechanical properties, permeability, dimensional stability, shutdown properties and meltdown properties, wherein the microporous polyolefin membrane comprises polyethylene, and polypropylene having a mass-average molecular weight of 5×105 or more and a heat of fusion of 90 J/g or more when measured by differential scanning calorimetry, and wherein the polypropylene content is 20% or less by mass. JP2004-196871 A proposes a microporous polyolefin membrane comprises polyethylene, and polypropylene having a mass-average molecular weight of 5×105 or more, and a melting point of 163° C. or higher when measured by differential scanning calorimetry at a temperature-elevating speed of 3 to 20° C./minute, wherein the polypropylene content is 20% or less by mass. However, the microporous polyolefin membranes of these references do not have sufficient SD property.
JP2002-321323 A proposes a microporous polyolefin membrane having excellent safety and strength, wherein the microporous polyolefin membrane has an integrally laminated three-layer structure of A/B/A or B/A/B, and wherein A represents a microporous membrane comprising polyethylene and polypropylene as indispensable components, and B represents a microporous polyethylene membrane. However, all Examples in this reference are directed to microporous membranes having a three-layer structure of A/B/A, failing to show the three-layer structure of B/A/B. In addition, since polypropylene in the microporous membrane A does not have optimized properties, this microporous polyolefin membrane can not necessarily have satisfactory SD property.